An image reading device includes a glass having a document reading surface along which a document moves; an image reading unit movable in a sub-scanning direction and including a light emitting portion that emits light to illuminate the document with the light and a light receiving portion that receives the light reflected by the document, the image reading unit reading the document that moves along the document reading surface at a predetermined document reading position; and a light emission control unit that causes the light emitting portion to emit the light so that the light enters the glass through a side surface that crosses the document reading surface in an operation different from an operation in which the image reading unit reads the document.